This application is based on and claims the priority under 35 U.S.C. xc2xa7119 of German Patent Application 200 04 499.0, filed on Mar. 14, 2000, the entire disclosure of which is incorporated herein by reference.
The invention relates to aerodynamic components in general and particularly to helicopter rotor blades having a leading edge flap that is driven by a piezoelectric actuator.
In rigid wing aircraft, three spatially separated systems are conventionally used to control the lift, the propulsion, and the steering. Contrary thereto, in a helicopter the main rotor substantially assumes itself the above-mentioned control functions to assure a controlled flight. As a result, the design and construction of a helicopter rotor blade requires numerous compromises due to the non-stationary events that occur during flight of a helicopter. Thus, it is in the nature of a helicopter blade that its aerodynamics are not optimal in certain sections or areas along the blade if the rotor blade construction is fixed. For example, one aspect that is not optimal is the possible dynamic stall when the rotor blade is rotated in the reverse direction.
When a helicopter travels in the forward direction, the rotor blades are exposed along their leading edge to higher flow-on velocities than are effective on the trailing edge due to the vectorial superposition of the flight velocity and the rotational velocity. In order to assure a symmetrical distribution of the lift, the angle of attack of the profile of the rotor blades is cyclically varied for each full revolution of the blade. The faster the helicopter moves in the forward direction, the more angling must be applied to the blade profile along the trailing edge. However, at a certain point such steep angling leads to dynamic stall along locally limited areas of the rotor blade.
On the one hand, the dynamic stall may cause strong vibrations. On the other hand, the dynamic stall limits the performance of the helicopter. Moreover, the flight comfort is noticeably reduced for the pilot and the passengers by the noise caused by the blades and by the vibrations in the passenger cell. Moreover, such vibrations lead to a premature tiring of people and to fatigue failures in the materials of which the helicopter components are made. As a result, an increased inspection and maintenance effort and expense is unavoidable.
The problem can be alleviated by a continuous adaptation of the aerodynamic profile of the rotor blades to the continuously varying aerodynamic operating conditions, for example by means of a shape variable profile geometry. Such a shape variable profile geometry can significantly improve the performance, safety and comfort of helicopters. Starting from different structural possibilities of dynamic lift aids, it is known that a dynamic variation of the leading edge of the flow profile is aerodynamically very effective. For example, high suction peaks along the leading edge of the blade can be reduced by lifting and lowering of a leading edge flap also referred to as nose flap secured to the leading edge of the blade. Such lifting and lowering of the nose flap delays the dynamic stall or flow separation and reduces the hysteresis loops in the course of the aerodynamic coefficients. Additionally, a discrete nose flap makes it possible to provide the required energy for overcoming the aerodynamic forces and moments required for a continuous variation of the contour by an elastic deformation or to provide a larger motion range for these contour deformations.
U.S. Pat. No. 5,409,183 (Gonzales) discloses a helicopter rotor blade with leading edge servo-flaps for pitch positioning the rotor blade. The rotor blade is equipped with a flap that extends in front of the leading edge of the blade because the flap is mounted on two brackets (42) secured to the fixed rotor blade (28). The flap is tiltable or movable about an axis (40) by a hydraulic actuator (44) through a linkage and bellcrank mechanism (46, 48). Such a system has the drawback that the relatively complicated mechanical linkage mechanism does not provide the required high adjustment speeds in response to the control signals. Additionally, the linkage requires a plurality of pivot joints which makes it prone to a high repair requirement. Moreover, such a linkage displaces with its own weight the center of gravity distribution in the rotor blade, thereby causing unfavorable center of gravity conditions.
An article entitled xe2x80x9cDevelopment of High Performing Piezoelectric Actuators For Transport Systemsxe2x80x9d, published at a xe2x80x9cActuator 98xe2x80x9d meeting on Jun. 17 to 19, 1998 in Bremen, Federal Republic of Germany, describes a helicopter rotor blade having a journalled servo-flap functioning as a trailing edge flap which is adjusted by piezoelectric actuators. These piezo-actuators are distributed in the rotor blade in the longitudinal direction thereof, that is in the span-width direction of the blade. Arranging the piezo-actuators in the span-width direction for driving a leading edge flap would take up a substantial proportion of the available span-width or blade length. Additionally, such a longitudinal distribution of the piezoelectric actuators along the leading edge of the rotor blade would adversely influence the center of gravity distribution in the rotor blade. A pair of piezo-actuators disposed or displaced in the span-width direction furthermore would not be able to apply to the leading edge flap the forces required due to the high air loads, without twisting the leading edge flap.
U.S. Pat. No. 5,626,312 (Head) discloses a piezoelectric actuator or transducer (1) in the form of a torque rod (15, FIG. 3). The torque rod (15) is constructed of a plurality of piezoelectric rod elements (8) that are connected at their ends to end plates (4) and (6) to form a squirrel cage that is installed in a helicopter blade (31) in the lengthwise direction for operating a leading edge flap or a trailing edge flap or both, please see FIGS. 6A and 6B of Head. Due to the lengthwise orientation of the transducer (1) in the rotor blade a substantial installation space is required. Moreover, a substantial number of rod elements (8) forming the squirrel cage contributes to the weight of the blade.
In view of the above it is the aim of the invention to achieve the following objects singly or in combination:
to construct an aerodynamic component such as the helicopter rotor blade or a wing or the like having a flow profile with a leading edge flap, in such a way that the flap can be adjusted with a high adjustment speed, with high accuracy, with a rapid response characteristic and without play;
to construct a piezo-actuator for such high speed adjustment which requires an optimally reduced maintenance and repair;
to construct the piezo-actuator in such a way that its piezo-elements can be arranged in the chord direction of the aerodynamic component rather than in the span-width or length direction, to thereby avoid changing the center of gravity characteristics of modern rotor blades;
to provide the above-mentioned high speed adjustments even for extreme flight maneuvers and conditions;
to avoid a plurality of pivoted rod linkage and bellcrank elements as well as torque generating piezoelectric elements that require a substantial installation length;
to construct a piezo-actuator for adjusting a flap on an airfoil with an optimal performance while the actuator has a minimal weight; and
to construct the actuator in such a way that the flap adjustment is accomplished by a push-pull action.
A component having an aerodynamic flow profile such as a helicopter rotor blade having at least one leading edge flap or nose flap along its leading edge is characterized according to the invention in that each nose flap is driven by a piezo-actuator which comprises two piezo-elements forming a pair. The piezo-elements are arranged one behind the other in the chord direction of the rotor blade. The two piezo-actuator elements are secured to a fixed point positioned between the piezo-elements. The fixed point is part of or fixed to the blade body. The power output of the two serially arranged piezo-actuator elements is transmitted to the nose flap by a push rod and by a two-pronged member referred to herein as fork. One end of the push rod is operatively connected to the piezo-actuator element closer to the trailing edge of the rotor blade. The two-pronged member is operatively connected to the piezo-actuator element positioned closer to the leading edge of the blade. The other end of the push rod and the other end of the two-pronged fork are operatively connected to a free end of a lever which in turn is rigidly connected to a journal of the leading edge flap for operating the leading edge flap by a push-pull action.
Advantages of the invention are seen in that the push-pull action is a rapid action which provides the required adjustment speeds of the leading edge flap even for extreme flight maneuvers. Further, the arrangement of the piezo-actuator elements in the chord direction provides a very compact construction which does not disturb the center of gravity distribution of the blade. More specifically, the center of gravity distribution in a conventional rotor blade is maintained where the present actuator take up 25% of the relative profile depth. Another advantage of the invention is seen in that the integration of the piezo-actuator elements into the rotor blade in the chord direction avoids a mechanical linkage that requires a substantial number of link elements, bellcranks and detouring pivot joints which require a large installation space and do not act fast enough. The piezo-elements arranged in the chord direction permit a compact construction which has the added advantage of requiring little maintenance work. Moreover, the present construction satisfies the requirements that are to be met especially for helicopter rotor blades, namely a low profile that requires a small installation space, that can take up high centrifugal forces, and provides a high performance while simultaneously being lightweight. The actuator characteristics of piezo-ceramic elements are based on the inverse piezoelectric effect. More specifically, piezo-ceramic elements having such an effect are solid bodies which change their dimensions in response to an exposure to a directed electrical field. Due to such a solid body characteristic a very high force level, a high precision, and a high dynamic or response speed can be achieved without play. Piezo-actuators of this type comprise a multitude of separate piezo-layers which are stacked to form a column of piezo-layers with electrodes interposed between the piezo-layers to contact these layers for the application of the required energizing voltage. The electrodes are selectively connected to voltages having opposing polarities. The piezoelectric layers expand and contract in the longitudinal direction of the column or stack. As a result, the expansions and contractions are summed in response to the electrical energizing of the individual piezo-layers, whereby the obtainable output of the solid piezo-column is substantially increased to provide a high performance density, so to speak, which makes it possible to completely integrate the piezo-actuators into the rotor blade contour even in the chord direction. Merely three electrical conductors are required for the energy supply length-wise through the rotor blade. The supply of electrical energy to the three conductors takes place at the rotor drive shaft or mast through conventional slip or collector rings.
According to the invention the power generated by the expansion and contraction of the piezo-elements in the blade length direction, is preferably redirected by respective force direction changing frames, each of which surrounds its piezo-element.
More specifically, the redirection of the power output from the first piezo-element positioned closer to the trailing edge of the blade to the noseflap is accomplished by the above-mentioned push rod which passes directly through both piezo-elements and through both force direction changing frames, whereby the push rod is guided through holes in the piezo-elements or through holes in spacer elements. The redirection of the power output of the second blade positioned closer to the leading edge, to the nose flap is accomplished by the above fork connected with one end of its two prongs to the respective force direction changing frame.
Instead of using a push rod and a fork acting as a pull element, two forks could be used. The prongs of one of the forks for transmitting the output of the rearwardly positioned piezo-element would bypass both piezo-elements thereby requiring more space in the chord depth or blade height direction. Therefore, and due to the required bending of the prongs of the one fork and due to the limited strength of materials for making the forks, it is preferred to use a push rod for the transmission of the piezo-expansion from the rearwardly positioned piezo-element to the leading edge flap and a fork for the transmission of the piezo-expansion from the forwardly positioned piezo-element. The arrangement of the push rod passing through the first and second piezo-element requires little space and achieves a small structural height or low profile in the blade depth direction while simultaneously providing a symmetric force introduction into the leading edge flap from the piezo-actuator elements forming a pair.
The holes in the piezo-elements are simply drilled directly into the piezo-elements. However, it is preferred to construct the piezo-elements of two sections each and to insert a spacer member between the two piezo-sections of a piezo-element. The holes for passing and guiding the push rod in the chord direction through the piezo-elements are provided in the spacer elements. The hole diameter provides a guided sliding fit of the push rod through the respective spacer member. The spacer members are made of metal. However, due to the specific material characteristics of lithium fluoride (LiF) that material is preferred for making the spacer members. In addition to guiding and spacing the push rod from the piezo-elements the spacer members also perform a temperature compensation function because the piezoelectric elements have a smaller temperature expansion coefficient than the metal components that surround the piezo-elements for the force transmission as will be described in more detail below. The temperature compensation by the spacer members prevents or reduces any performance loss of the actuator system in response to heat. Another advantage of the spacer members is seen in that it is not necessary to provide a hole in the stack or column of piezo-layers. Instead, two stack sections are simply spaced from each other by the spacer members which facilitates the arrangement of the electrodes in the stack sections because holes need not to be drilled where electrodes might otherwise be positioned. Since the spacer members are not made of piezo-material, there is a minute power or performance reduction which are easily compensated by respectively dimensioning the piezo-elements in the actuator and selecting the number of piezo-layers needed for obtaining a required output in the form of the dimension of the expansion and contraction of the two piezo-columns.
The arrangement of the piezo-elements so that the respective column is physically oriented in the blade length direction and so that the respective expansion and contraction also takes place in the blade length direction, has the advantage that disturbing effects caused by the high acceleration forces prevailing in an operating helicopter rotor blade can be minimized, since no force transmitting motions of the push rod and fork take place crosswise to the acceleration direction of the blade. The force transmission for the adjustment motion of the position of the nose flap from the piezo-elements to the flap takes advantageously place in the chord direction and hence the acceleration direction, whereby the direction transformation redirection of the force is accomplished by the above mentioned push-pull arrangement connected to the respective piezo-column by the force direction changing frames.
Preferably, these force direction changing frames are solidly constructed frames, preferably having a rhombic or rectangular frame configuration surrounding the respective piezo-element or column. Each frame comprises frame sections preferably in the form of leaf springs which are interconnected through pivot joints preferably integrally formed in the frame sections for interconnecting the frame sections to form a four-bar linkage which kinematically transforms the expansion and contraction of the respective piezo-element into a force directed perpendicularly to the expansion direction and thus in the chord direction. The force direction transforming frame increases or amplifies the expansion distance of the piezo-element whereby the respective force is reduced in accordance with the law of levers. Such a force transformation frame structure as just described is absolute free of play and any loss of energy in the bending zones of the frame sections are negligibly small. Thus, the frame structure with its pivot joints assures a coupling free of play between the respective piezo-element and the leading edge flap so that canting of the individual frame sections is prevented even at high acceleration forces.
If the force transformation frame, the push rod and the pull fork are used in combination, a further advantage is achieved in that the push-rod can also pass through both force transformation or force direction changing frames to transmit the expansions from the first or rear piezo-element forwardly to the leading edge flap. Providing the frame sections with the required holes for the passage of the push rod does not pose any problems because the frame sections are preferably made of a metallic material. mentioned, the two piezo-elements forming a pair are arranged one behind the other in the chord direction. The piezo-elements of the pair are structurally secured to the body of the rotor blade by a fixed point positioned between the two members of the pair of piezo-elements. A mechanical bias force applied to the piezo-elements is advantageous in the operation of piezo-elements for preventing the application of tension loads to the piezo-elements. Such mechanical tension force can be applied by adjustment screws which may serve simultaneously for the geometric fine positioning of the leading edge flap. In an alternative embodiment the biasing of the piezo-elements may be accomplished by springs which are arranged around the piezo-elements. However, it is particularly advantageous to interconnect the piezo-elements electrically in such a way that they operate in a push-pull fashion. Thus, when one piezo-element of a pair expands, the other contracts and vice versa. The force transmission is then performed in such a way that one piezo-element tilts the leading edge flap downwardly while the other piezo-element turns the flap upwardly. In this manner the piezo-elements are not exposed to any tension loads, whereby the above-mentioned mechanical biasing force is applied by one piezo-element to the other and vice versa.
The biasing of the piezo-elements can additionally be increased electrically by a so-called offset voltage applied to the piezo-elements. The value of the offset voltage corresponds advantageously to one half of the maximum voltage that may be applied to the particular piezoelectric elements in accordance with specifications provided by the manufacturer of these elements. The effect of the offset voltage causes a certain deflection or expansion of the piezo-elements, thereby biasing each other.
Under certain flight conditions it may be necessary to tilt the nose flap upwardly and downwardly at different times relative to the central chord line. However, due to aerodynamic considerations it is preferred to tilt the nose flap only downwardly relative to the central chord line and up again only to the central chord line at which the nose flap assumes a neutral position. Any motion of the nose flap upwardly above the central chord line is prevented by a respective stop. Due to this stop the piezo-elements do not have to expand for the purpose of moving the flap upwardly. As a result a high blocking force for keeping the leading edge flap in this neutral position can be applied by a stationary stop secured to the body of the blade. Due to aerodynamic reasons the neutral position of the flap is primarily required in the forwardly rotating rotor blade.
The nose flap is preferably so-constructed that it forms the leading edge of the blade including its flow profile along the entire length of the noseflap. Such a structure does not differ aerodynamically in its profile in the neutral position of the nose flap from a conventional flow profile constructed for the same purpose. There may be situations, such as structural conditions, making it necessary to journal the nose flap outside of the respective blade section contour or profile. However, the integration of the nose flap into the blade section provides an aerodynamically more advantageous profile or contour when the nose flap is in its neutral position and when it is in its deflected position. Another advantage of integrating the nose flap into the contour or profile of the blade is seen in that such integration is structurally simpler than nose flaps that are journalled outside the blade. Additionally, the integration also facilitates the exchangeability of the nose flap and thus of the erosion protection provided along the leading edge. In other words, the erosion protection may be provided also along the outwardly facing surfaces of the nose flap.
Another advantage of the structure according to the invention is seen in that the energization and the power output of the piezo-elements provides the required high adjustment speeds in combination with a high force level without friction and without play. These advantages are most effectively utilized if the bearing to which the nose flap is journalled to the blade section is a precision anti-friction bearing such as a roller bearing or bearing box.